Media Memo

Scientists working with the Hopkins Ultraviolet Telescope on the Astro-2 mission aboard the space shuttle Endeavour are marveling at yet another instance of uncanny timing.

This time the news concerns cataclysmic variables, binary star systems made up of two stars in orbit around each other. One star is an extremely massive but compact star called a white dwarf. The other is a more normal star, with less mass than our own Sun.

Cataclysmic variables go into periods of "outburst," during which they increase dramatically in brightness, followed by long periods of dormancy.

Wednesday morning, astronomers believe, they observed and recorded precise spectrographic data from a cataclysmic variable that appears to be on the brink of going into a period of outburst.

The timing could not have been better. Astronomers had observed the same cataclysmic variable on the Astro-1 mission, in 1990, right after it had finished going through an outburst, and had measured its temperature at about 37,000 degrees.

Scientists observing the object Wednesday said its temperature is drastically cooler before outburst than after outburst.

Researchers are eager to begin analyzing data from the ultraviolet telescope, which will enable astronomers to make precise temperature measurements, yielding insights into the physics behind cataclysmic variables.

The cataclysmic variable, called U Geminorum, is about 250 light years from Earth, in the constellation Gemini, said William Blair, a Johns Hopkins astrophysicist who is involved in the work along with astronomer Knox Long from the Space Telescope Science Institute in Baltimore.

U Gem, as astronomers call it, usually takes about four months to go into outburst and apparently is on the verge of doing so now.

Also Wednesday, the Hopkins team observed another cataclysmic variable, called SS Cygni, in the constellation Cygnus, about 300 light years away.

Astronomers believe the mysterious objects go into periods of outburst because of matter from the low-mass star falling into the white dwarf. The two stars are locked together by powerful gravitational attraction in very close orbits around each other. The white dwarf is a star about the size of Earth but as massive as the Sun -- it has burned all of its nuclear fuel and shrunk to the size of a planet, resulting in matter so dense that a piece the size of a golf ball would weigh 35 metric tons.

The other star sheds matter, which apparently is captured by the gravity of the white dwarf. Scientists believe the matter may accumulate in a disk around the white dwarf. Eventually, the disk apparently accumulates enough mass to cause it to dump a load of matter onto the white dwarf. The energy released causes the outburst.

The binary star systems are ideal laboratories for observing and understanding the process of accretion, whereby material falls onto an object of great mass. Astrophysicists are trying to learn more about accretion because it is thought to be essential for the formation of planets, stars and many other astrophysical situations. Scientists believe that accretion of material onto a massive black hole may be the power source for quasars as well, although on a much different scale than in binary stars. However, insights into the physical processes of heating and ionization of material in these binary stars may have applications to these other diverse physical situations.

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